1. Field of the Invention
The present invention relates to a flyback converter with synchronous rectifier to simply the circuit design, and improves the power efficiency by using a current sensor to detect the secondary current of the transformer to control the duty cycle of the synchronous switch.
2. Background of the Invention
Recently, the technique of the switching power supply is getting more progress. Operating in coordination with the variation of the circuit topology, the switching power supply has been an essential technique in the computer and electronic equipment power source. The switching power supply is a popular device because it provides the power source with the stability, compactness, efficiency and the lower cost for computer, rather than the traditional linear transformer circuit.
In the switching power supply area, the synchronous rectify method has been use for many years. Generally, the synchronous rectify method is almost applied in the forward converter or the resonant converter. FIG. 1 is the prior art U.S. Pat. No. 5,991,171, uses the synchronous rectify technique. The Synchronous rectifier control 122 connected in the secondary circuit of a forward DC-to-DC converter, not in a flyback converter.
FIG. 2 is the prior art U.S. Pat. No. 5,991,172, presents a flyback converter reduced the turn-on switching losses in a single stage by using a zero-crossing detector device to control the SW operation. Obviously, FIG. 2 is not using the synchronous rectify technique in the flyback converter. The main disadvantage of U.S. Pat. No. 5,991,172 is more complex circuit design to drive the SW. On the other hand, the zero-crossing detector device equipped for the primary circuit and the feedback signal was the divided voltage. The present invention would be very different from FIG. 2 by controlling the current feedback level and detecting the secondary current with synchronous rectifier in the flyback converter.
As mention above, it is obviously that the synchronous rectifier never be applied into the flyback converter. The reason is that the characteristic of the flyback circuit is often interrupted by the parasitic capacitor and leaking inductance operation to make the current switching waveform more complex. The parasitic capacitor and leaking inductance interruption result in getting the synchronous control signal hardly, so there is not use the synchronous rectifier in the flyback converter. Further, some producers use much more complex controlling method to control the switch in good performance. But the flyback switching power supply is using extensively with the consideration of the low price and the simplificative circuit design. We should not use the complex controlling method in the flyback converter. Then this invention would provide the flyback converter with low cost and simplified circuit design.
The present invention relates to a flyback converter with synchronous rectifier by using the current sensor coupled with the synchronous rectifier to detect the secondary current of the transformer. Using the secondary current, this invention could control the pulse width of the output signal from the synchronous rectifier circuit to make the synchronous switch turn on in the same time with the diode. It also means the duty cycle of the synchronous switch driving signal can be controlled by the current feedback. The synchronous rectifier could reduce the component stress of the diode coupled with the synchronous switch.
The primary objection of this invention is to provide a flyback converter that improves the power consumption efficiency of the power supply and to avoid the secondary current flows back. This invention further simplifies the circuit design to make the production much easier, so the circuit production can be cost down rather than the traditional flyback converter.
In order to achieve the purpose described above, the flyback converter with synchronous rectifier in this invention embodiment comprising a power source which provide the power for the flyback converter. The power source coupled to a flyback switch circuit for switching, then output a high frequency pulse to a transformer. The transformer has a primary coil coupled to the flyback switch circuit to receive the high frequency pulse, and has two secondary coils for one is master source and the other is sub-source. The sub-source is connected to a synchronous rectifier via a diode D1 to provide power for synchronous rectifier circuit. The synchronous rectifier output is a driving pulse. The driving pulse would drive a synchronous switch that is parallel coupled with an output diode. The output diode is placed between the master source and the load. The load further connects with an output capacitor in parallel. There is also a current sensor coupled to the load in series connection to detect the load current, further the current sensor transmitted the load current into the synchronous rectifier for comparing with a preset current level to regular the driving pulse to meet what we need.
Wherein, the current sensor detect the load current to make the synchronous rectifier output a regulated driving pulse when the output diode is on, then the synchronous switch turn on with the output diode simultaneously to bypass the current of output diode for reducing the component stress and switching loss.
For the preferred embodiment, further including a promoted turn on/off path coupled between the sub-source and the gate electrode of synchronous switch. The path includes a quickly turn on diode D5 connected with a quickly turn off diode D6, and the quickly turn on diode D5 coupled with a capacitor C5 in parallel connection. In the meantime, the preferred embodiment further including a buffer consisted of an N-type transistor Qn connected with a P-type transistor Qp. Both the base electrodes of the Qn and Qp are connected together then coupled to the output of the synchronous rectifier. Both the emitter electrodes of the Qn and Qp are also connected together then coupled to Vs and to the gate electrode of synchronous switch. The collector electrode of Qn coupled to the node N to connect with the promoted turn on/off path, the collector electrode of Qp coupled to the Vt.
Wherein the promoted turn on path consists of the diode D5, capacitor C5, Qn and the gate of the M1 used to form a quickly charging circuit for the synchronous switch M1. The promoted turn off path consists of the diode D6, capacitor C5 used to form a quickly discharging circuit for the synchronous switch M1.